Preparation of esters of 2,6- and 2,7-naphthalene dicarboxylic acid

ABSTRACT

ESTERS OF 2,6-AND 2,7-NAPHTHALENE DICARBOXYLIC ACIDS ARE PRODUCED BY (1) CONCENTRATING A MIXTURE OF 2,6-AND 2,7-NAPHTHALENE ISOMERS BY FRACTIONAL CRYSTALLIZATION, (2) OXIDIZING THE ISOMERS TO THE CORRESPONDING DIACIDS, (3) ESTERIFYING THE MIXED ACIDS, AND (4) SEPARATING THE ISOMERIC ESTERS BY CRYSTALLIZATION.

United States Patent 3,565,945 PREPARATION OF ESTERS 0F 2,6- AND 2,7- NAPHTHALENE DICARBOXYLIC ACID Earl W. Malmberg, Wilmington, Del., and Richard P.

Barbor, Malvern, Pa., assignors to Sun Oil Company, Philadelphia, Pa., a corporation of New Jersey Continuation-impart of application Ser. No. 154,292,

Nov. 22, 1961. This application June 24, 1966, Ser.

Int. Cl. C07c 69/76 US. Cl. 260-475 6 Claims ABSTRACT OF THE DISCLOSURE Esters of 2,6- and 2,7-naphthalene dicarboxylic acids are produced by (1) concentrating a mixture of 2,6- and 2,7-naphthalene isomers by fractional crystallization, (2) oxidizing the isomers to the corresponding diacids, (3) esterifying the mixed acids, and (4) separating the isomeric esters by crystallization.

This application is a continuation-in-part of application Ser. No. 154,292, now abandoned, filed Nov. 22, 1961, by the present inventors.

This invention relates to a process for the production of esters of 2,6- and 2,7-naphthalene dicarboxylic acid. More particularly, in one embodiment, the invention relates to a process for the production of 2,6- and 2,7-dimethylnaphthalate. More particularly, the invention relates to a process for the production of 2,6- and 2,7- dimethylnaphthalate from concentrated charge stocks derived from petroleum fractions boiling in the 400550 F. range.

There are ten possible dimethylnaphthalene isomers; and most, if not all of these, occur in charge stocks boiling in the 400-550 F. range. Due to the close boiling points of these isomers, the separation from the mixture of any specific isomer in high concentration is a difficult task. Recently there has been an interest in the diesters of naphthalene dicarboxylic acids for use in making polyester fibers. The principal interest has centered around the 2,6-isomer, but the 2,7-isomer also has potential in this and other areas of chemical processing. There are indications that some mixtures of 2,6- and 2,7-isomers have polymerization value.

We have found that the 2,6- and 2,7-isomers can be processed together through a series of sequential steps and separated as diesters by fractional crystallization. Broadly speaking, the process of the invention comprises the steps of (l) distilling a petroleum fraction and separating a heart-cut fraction boiling in the range of 500-510 F. which contains a major proportion of dimethylnaphthalenes, (2) fractionally crystallizing the latter fraction to separate a 2,6- and 2,7-dimethylnaphthalene concentrate, (3) oxidizing the concentrate to obtain 2,6- and 2,7-diacids, (4) esterifying the diacids, and (5) separating the diester of 2,6-naphthalene dicarboxylic acid and the diester of 2,7-naphthalene dicarboxylic acid by fractional crystallization.

The method of practicing the present invention is illustrated in the drawing, which shows the steps outlined above. The feed preparation step will be discussed more fully below.

Hydrocarbon fractions known as gas oils fboiling within the range of from about 400 to about 550 F. are obtained in both catalytic and thermal cracking processes as a part of the over-all petroleum refining process. Gas oils contain substantial amounts of alkyl naphthalenes, including mono-, di-, and tri-methylnaphthalenes and in smaller quantity, the ethyl naphthalenes. Such fractions Patented Feb. 23, 1971 typically may have aromatic contents varying within the range of 25-97 percent but usually contain between 50 percent and percent aromatics, depending upon the particular operation in which the petroleum fractions are produced. The aromatics can be concentrated by solvent extraction, for example, with furfural.

Stocks having high alkyl naphthalene contents can also be obtained by extracting straight-run petroleum fractions of appropriate boiling ranges, such as kerosene and the like, with solvents, such as furfural or sulfur dioxide, or by selective adsorption. Stocks having a high sulfur content can be hydrodesulfurized.

The undesired alkyl naphthalenes such as the monoalkylnaphthalenes and 1,3-, 1,6-, and 1,7-dimethylnaphthalenes can be subjected to isomerization, alkylation, dealkylation, and disproportionation to provide additional quantities of 2,6- and 2,7-dimethylnaphthalene concentrate for treatment in the process of the invention. Furthermore, the desired isomers can be separated from cycle or by-product stocks obtained in processes directed primarily to the production of naphthalene.

Following feed preparation, the feed stock comprising an alkyl naphthalene aromatic fraction boiling above about 500 F. is fractionated under efiicient conditions in a tower employing about 30-50 theoretical plates and reflux ratios of the order of 30:1 to 50:1. Under these conditions the content of 2,6- and 2,7-dimethylnaphthalenes in the 500-510 F. fraction will be of the order of 30-50 percent. A typical composition of this fraction is:

Percent l-EN 2 2,6-DMN 24 2,7-DMN 22 1,3-DMN 13 1,7-DMN 17 1,6-DMN 14 1 Ethyl naphthalene. 2 Dimethylnaphthalene.

The 500-510 P. fraction is passed to crystallizing and filtering facilities. It is chilled to a temperature in the range of from about --10 to 30 F. and filtered at this temperature. The filter cake is preferably subjected to partial melting at a temperature in the range of from about 20-85 F. While pressing to remove the melt. The residual filter cake has a 2,6- and 2,7-dimethylnaphthalene content of 60-99 percent. The filtrate can be returned to the feed preparation zone for further processing. More than one stage of fractional crystallization can be used. A solvent can be used if desired.

The oxidation step can be accomplished by either of two processes which have been found to be satisfactory for the production of naphthalene dicarboxylic acids, particularly the 2,6- and 2,7-isomers or mixtures thereof.

In the first process the dialkyl naphthalene feed is dissolved in a suitable solvent and treated with gaseous N0 Suitable solvents include the chlorobenzenes, particularly di-, tri-, and tetra-chlorobenzene. The oxidation is conducted at temperatures ranging from to 225 C. High yields are achieved in the presence of a selenium catalyst, but the oxidation can be conducted without a catalyst.

EXAMPLE 1 Two grams of selenium substantially dissolved in 1,500 ml. trichlorobenzene were heated to C. with stirring in a 3-liter resin flask. N0 was introduced at the rate of 1.0 to 1.5 grams per minute. Two hundred grams of mixed 2,6- and 2,7-dimethylnaphthalenes from fractional crystal lization were dissolved in 500 ml. of trichlorobenzene, and the solution was added incrementally to the flask. The temperature was maintained at about 200 C. for about 350 minutes.

The contents of the flask Were cooled, the solids separated by filtration, Washed with pentane, and dried. The diacid product weighed 272 grams and had an acid number of 498 corresponding to 90-93 percent diacid (theoretical- 519). The yield was approximately 85 percent based on the hydrocarbon feed.

An alternative oxidation procedure comprises contacting a mixture of 2,6- and 2,7-dimethylnaphthalene dissolved in a 2- to 4-carbon-atom fatty acid solvent with an oxygen-containing gas in the presence of cobaltic ions at a temperature in the range of from about 90 C. to about 140 C. and recovering the diacid from the resulting reaction products. The preferred temperature range is 110 to 135 C. Operating pressure is not a critical factor; and therefore, atmospheric pressure is preferred unless a higher pressure is needed to maintain liquid phase reaction conditions. Pressures of to 500 p.s.i.a. are suitable. Molecular oxygen is the desired oxidizing agent. Cobalt acetates and propionates are the preferred catalysts. Acetic acid and propionic acid are the preferred solvents. Reaction times are from 6 to hours. The reaction is preferably conducted in the presence of a bromide activator.

The mixed 2,6- and 2,7-naphthalene dicarboxylic acids from the oxidation step are esterified in the presence of an alcohol and acidic catalyst. Low molecular weight aliphatic alcohols containing 1 to 6 carbon atoms per molecule such as methanol, ethanol, n-propyl and isopropyl alcohol are suitable esterification agents. Methanol is preferred. The weight ratio of alcohol to diacid should be in the range of from 7.0 to 1 to 20.0 to 1. The preferred operating range is 7.5 to 1 to 8.5 to 1. Inorganic acids are suitable catalysts. Examples are H 80 HCl, H PO BP and HF. Trifluoroacetic acid can also be used. The preferred members of this group are HCl and H 80 The latter can be used in concentrations of 75 to 100 percent or more, preferably 75 to 85 percent. The catalyst is employed in amounts of 1 to 20 percent by weight based on the mixed diacid isomer feed. Temperatures in the range of to 170 C. are suitable but not limiting, since higher or lower temperatures may be used. Pressures can range from atmospheric to 300 p.s.i.g. with a range of 50-150 p.s.i.g. being most suitable. Times of 1-6 hours are satisfactory.

EXAMPLE 2 A glass-lined reactor equipped with a stirrer was charged with 451 grams of mixed 2,6- and 2,7-naphthalene dicarboxylic acids having a ratio of 58 percent 2,6-isomer and 42 percent 2,7-isomer. Eight thousand grams of methanol and 45 grams of 80 percent H SO were added. The reaction mixture was heated to and maintained at a temperature of 260265 F. and a pressure of about 140 p.s.i.g. After 4 hours the reaction mixture was cooled to room temperature and filtered. The filter cake weighed 360 grams and consisted of 70 percent 2,6-diester and 30 per cent 2,7-diester. An additional 125 grams of diester mixture were recovered from the esterification mother liquor.

The dimethyl esters of 2,6- and 2,7-naphthalene dicarboxylic acids are separated from one another by fractional crystallization. The products are usually recovered directly from the esterification reaction mixture. If de sired, the methanol and catalyst may be removed from the mixed diesters; however, we have found that this is not necessary. A feature of the invention is that high yields of high purity isomers can be obtained by cooling the esterification reaction mixture. The fractions containing a high proportion of 2,6- or 2,7-isomer can then be recrystallized with another solvent. Suitable recrystallization solvents are those boiling above about 100 C. Examples include aliphatic ketones (methyl ethyl ketone), phenyl ethers (anisole), low molecular weight aliphatic alcohols having 1 to 6 carbon atoms (butyl alcohol), low molecular weight aliphatic esters (butyl acetate), and nitrogen ring compounds such as pyridene.

Suitable aromatic recrystallization solvents include benzene, toluene, xylenes, durene, cumene, and ethylbenzene. Benzene and toluene are preferred.

In one embodiment the esterification reaction mixture is cooled to a temperature in the range of from about 15 to 35 C., preferably 20 to 30 C., to crystallize a first fraction containing 65-75 percent 2,6-diester and 3525 percent 2,7-diester after one or more washings with methanol. A second fraction is obtained by diluting the methanol washings with water. This fraction contains about 90 percent 2,7-diester nad 10 percent 2,6-diester. The mother liquor is treated by the addition of about 3 parts of water to 4 parts of mother liquor to crystallize additional diesters. This third fraction contains about 75 percent 2,7- diester and about 25 percent 2,6-diester. The first fraction, i.e., from the filter cake, is then dissolved in an aromatic hydrocarbon solvent. The solution is heated to a temperature in the range of from 25200 C. and cooled to a temperature in the range of from 1535 C. The filter cake contains 2,6-diester of more than 90 percent purity. Conventional crystallization and filtering equipment can be used in all steps.

At any stage in the separation prior to the addition of water, the mother liquor can be recycled to the esterification step or distilled for recovery of part of the meth anol. After the addition of water the mother liquor can be treated to recover the methanol.

The 2.6- or the 2,7-diester can be treated in any number of recrystallization stages to raise the purity of the predominating diester isomer to 99 percent purity. The purified esters can then be recombined in any desired proportion for further use.

EXAMPLE 3 Four hundred fifty-four grams of mixed 2,6- and 2,7- naphthalene dicarboxylic acids in a 1 to 1 ratio are dissolved in 7.26 kilograms of methanol and esterified in the presence of 45 grams of percent H 50 in the manner described previously. After four hours the reaction mixture is cooled to 25 C. A filter cake weighing 292 grams and containing 68 percent 2,6-diester and 32 percent 2,7-diester is recovered. Twenty grams of the filter cake was mixed with ml. of toluene and heated to C, The solution was cooled to 25 C., and a filter cake weighing 12.1 grams was recovered. The diester product contained 95 percent 2,6-isomer and 5 percent 2,7-isomer, The liquid was evaporated to dryness, and 5.19 of diesters were recovered which analyzed 90 percent 2,7-diester and 10 percent 2,6-diester. The mother liquor from the cooled esterification product was also evaporated to dryness with water washing, and the product weighed 123 grams of which 85 grams were 2,7- diester and 18 grams were 2,6-diester.

The principal impurities are the aldester, the monoaldehyde, the monoester, and the dialdehyde. As the example shows, these are efiectively removed by the crystallization step of the invention. The products were analyzed by infrared and melting-point determination.

In another embodiment the esterification reaction mixture is cooled from the reaction temperature (about C.) to a temperature in the range of from about 8090 C. A filter cake is recovered which analyzes 80 percent 2,6-diester (about 75 percent of the total 2,6-isomer) and 20 percent 2,7-diester (about 20 percent of the total 2,7-isomer). The solution is further cooled to 0 C., and a filter cake containing 75 percent 2,7-diester (about.60 percent of'the total) and 25 percent 2,6-diester (about 25 percent of the total) is recovered. The remaining solution is washed with one-half volume of water and evaporated to dryness. The product contains 90 percent 2,7-diester (20 percent of the total) and 10 percent 2,6-diester (a few percent of the total). These fractions may be treated individually to further concentrate the predominant diester. The overall recovery represents 9095 percent of the 2,6-diester and about 90 percent of the 2,7-diester.

We claim:

1. A process for the preparation of the 2,6- and 2,7- diesters of naphthalene dicarboxylic acids comprising the steps of:

(1) concentrating a mixture of 2,6- and 2,7-dimethylnaphthalene isomers by fractional crystallization of a fraction boiling in the range of 500-510 F. containing a plurality of alkyl naphthaleneisomers,

(2) oxidizing the concentrated hydrocarbon isomers to produce a mixture of 2,6- and 2,7-naphthalene dicarboxylic acids,

(3) esterifying the mixed acids in the presence of an excess of a low molecular weight aliphatic alcohol,

(4) cooling the reaction product to crystallize the diester of 2,6-naphthalene dicarboxylic acid, and recovering the crystals, and

(5) recovering the diester of 2,7-naphthalene dicarboxylic acid from the mother liquor.

2. Process according to claim 1 in which the oxidation step is conducted with N0 3. Process according to claim 2 in which the oxidation is conducted in the presence of N0 and a selenium catalyst.

4. Process according to claim 2 in which the oxidation is conducted in the presence of cobaltic ions.

5. A process for the preparation of the 2,6- and 2,7- diesters of naphthalene dicarboxylic acids comprising the steps of:

(1) separating by distillation from petroleum stock a fraction boiling in the range of SOD-510 F. and containing a major amount of dimethylnaphthalenes,

(2) separating by fractional crystallization the 500- 510 P. fraction to separate a 2,6- and 2,7-dimethylnaphthalene concentrate from other dimethylnaphthalenes,

(3) oxidizing the said concentrate dissolved in a solvent by passing gaseous N0 into the solution,

(4) recovering a mixture of 2,6- and 2,7-naphthalene dicarboxylic acids,

(5) esterifying the mixture of 2,6- and 2,7-naphtha1ene dicarboxylic acids with menthanol in the presence of an inorganic acid,

(6) cooling the solution to a temperature in the range of 90 C. to crystallize the 2,6-diester, and recovering the filter cake,

(7) further cooling the remaining solution to a temperature of about 0 C. to crystallize the major proportion of the 2,7-diester and recovering filter cake,

(8) adding water to the remaining solution, and

(9) recovering an additional quantity of the 2,7-diester.

6. Process according to claim 5 in which each of the product fractions is extracted with another solvent to further concentrate the predominating diester isomer.

References Cited UNITED STATES PATENTS 3,219,691 11/1965 McNelis 260 -524 2,885,431 5/1959 Tarr 260 -457 2,968,674 1/1961 Franke et a1 260485 3,042,709 7/ 1962 Convery 260475 3,097,231 7/1963 Mills et al 260475 3,150,172 9/1964 Serres et a1. 260-524 FOREIGN PATENTS 823,437 11/1959 Great Britain 260-524N 1,080,999 5/1960 Germany 260524 JAMES A. PATTEN, Primary Examiner E. J. SKELLY, Assistant Examiner 

